Orthodontic brackets are widely used to realign teeth through the application of corrective forces provided by interconnected archwires. To provide proper adjustment of the teeth, orthodontic brackets are typically attached to each tooth requiring treatment either with a band or directly thereto by a bonding material, and are then interconnected with a tensioned orthodontic archwire.
In edgewise brackets, the archwire passes through a labially opening, horizontal arch slot and is prebent and/or otherwise shaped to provide the desired corrective force to each individual tooth. The slot is directly accessible in order to allow, for example, an archwire to be readily placed therein. Once placed in the slot, an archwire is typically restricted therein by some device. Some of the more common retaining devices are: ligature wires, elastomeric ligatures, and metallic pins. The more frequently used devices are metallic ligature wires and elastomeric ligatures.
The archwire slot generally passes horizontally through one pair of tie wings in a single bracket or two pair of tie wings in a double bracket. In each pair of tie wings one wing tip extends gingivally and one wing tip extends occlusally. Thus when the archwire is placed within the slot, a ligature or elastomeric ligature may be placed around one tie wing, over the archwire in the slot and then around the other tie wing. If there are two pairs of tie wings, such as with a standard "SIAMESE"-type or double bracket, a single ligature or elastomeric ligature can be selectively positioned around either or both pair of tie wings to impart desired rotational forces to the tooth.
In the orthodontic treatment of teeth, it is important for the bracket (and interconnected tooth) to be capable of desired movement relative to the archwire in response to the corrective forces applied thereby. For example, in many situations, frictional engagement between the metal or elastomeric ligature and the archwire may negatively effect the treatment plan of the orthodontist. Alternatively, in certain treatment regimes it may also be desirable to allow for but limit restrictive engagement to selected distal or mesial pairs of tie wings of an edgewise bracket.
Double edgewise brackets offer more options for such selective ligation than is possible with standard single brackets due to their spaced apart pairs of tie wings. That is, in a double bracket, a single ligature may be placed around both pairs of tie wings together, or a separate ligature may be placed around each pair of tie wings (i.e., one separate ligature per pair), or one ligature may be placed around only one pair of tie wings. Whichever ligation option is selected, the use of known brackets results in direct contact with the archwire wherever the ligature crosses the archwire. Direct contact tends to force the archwire into the archwire slot and, therefore, increases frictional contact with the bracket during movement of the tooth along the archwire. As previously stated above, frictional contact between the archwire and the bracket in the archwire slot may undesirably slow down the prescribed treatment. For proper treatment, as previously stated above, the bracket must move along the archwire as desired. While it is important to retain the archwire in the bracket slot, any undesired friction therebetween prolongs the time it is necessary to conduct treatment. Thus, there is a need for an edgewise double bracket that allows greater ligation selectivity and reduces or eliminates undesired frictional interference between the ligature and the archwire and between the archwire and the bracket.